Picosecond laser apparatus and methods for its operation and use

ABSTRACT

Apparatuses and methods are disclosed for applying laser energy having desired pulse characteristics, including a sufficiently short duration and/or a sufficiently high energy for the photomechanical treatment of skin pigmentations and pigmented lesions, both naturally-occurring (e.g., birthmarks), as well as artificial (e.g., tattoos). The laser energy may be generated with an apparatus having a resonator with the capability of switching between a modelocked pulse operating mode and an amplification operating mode. The operating modes are carried out through the application of a time-dependent bias voltage, having waveforms as described herein, to an electro-optical device (e.g., a Pockels cell) positioned along the optical axis of the resonator.

RELATED APPLICATION DATA

This application is a continuation application which claims priority toU.S. application Ser. No. 17/070,119, filed on Oct. 14, 2020, which is adivisional of U.S. patent application Ser. No. 14/708,828, filed on May11, 2015, which is a continuation of U.S. patent application Ser. No.12/534,379, filed on Aug. 3, 2009, which is a divisional of U.S. patentapplication Ser. No. 11/461,812, filed on Aug. 2, 2006, U.S. Pat. No.7,586,957, issued on Sep. 8, 2009, each of which are hereby incorporatedby reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to apparatuses and methods for deliveringlaser energy having a short pulse duration (e.g., less than about 1nanosecond) and high energy output per pulse (e.g., greater than about250 millijoules). The desired operating parameters are achieved throughthe application of a bias voltage, having a time-dependent value asdescribed herein, to an electro-optical device such as a Pockels cell.The Pockels cell may be disposed in a single laser having a resonatorthat can be operated in two modes, depending on the bias voltage appliedto the electro-optical device. As a result, laser energy suitable for anumber of applications, including treating and removing pigmentparticles such as those introduced to the human body as tattoos, may begenerated using a relatively simple apparatus.

BACKGROUND OF THE INVENTION

Lasers are recognized as controllable sources of radiation that isrelatively monochromatic and coherent (i.e., has little divergence).Laser energy is applied in an ever-increasing number of areas in diversefields such as telecommunications, data storage and retrieval,entertainment, research, and many others. In the area of medicine,lasers have proven useful in surgical and cosmetic procedures where aprecise beam of high energy radiation causes localized heating andultimately the destruction of unwanted tissues. Such tissues include,for example, subretinal scar tissue that forms in age-related maculardegeneration (AMD) or the constituents of ectatic blood vessels thatconstitute vascular lesions.

The principle of selective photothermolysis underlies many conventionalmedical laser therapies to treat diverse dermatological problems such asleg veins, portwine stain birthmarks, and other ectatic vascular andpigmented lesions. The dermal and epidermal layers containing thetargeted structures are exposed to laser energy having a wavelength thatis preferentially or selectively absorbed in these structures. Thisleads to localized heating to a temperature (e.g., to about 70° C.) thatdenatures constituent proteins or disperses pigment particles. Thefluence, or energy per unit area, used to accomplish this denaturationor dispersion is generally based on the amount required to achieve thedesired targeted tissue temperature, before a significant portion of theabsorbed laser energy is lost to diffusion. The fluence must, however,be limited to avoid denaturing tissues surrounding the targeted area.

Fluence, however, is not the only consideration governing thesuitability of laser energy for particular applications. The pulseduration and pulse intensity, for example, can impact the degree towhich laser energy diffuses into surrounding tissues during the pulseand/or causes undesired, localized vaporization. In terms of the pulseduration of the laser energy used, conventional approaches have focusedon maintaining this value below the thermal relaxation time of thetargeted structures, in order to achieve optimum heating. For the smallvessels contained in portwine stain birthmarks, for example, thermalrelaxation times and hence the corresponding pulse durations of thetreating radiation are often on the order of hundreds of microseconds toseveral milliseconds.

The use of even shorter pulses, however, results in a change fromphotothermal to photomechanical processes. The latter mechanism isinvoked by applying laser pulses having a duration that is below theacoustic transit time of a sound wave through targeted particles. Thiscauses pressure to build up in the particles, in a manner analogous tothe accumulation of heat within a target irradiated by laser pulseshaving a duration that is below the thermal relaxation time.

Photomechanical processes described above can provide commerciallysignificant opportunities, particularly in the area of treating skinpigmentations including tattoos, portwine stains, and other birthmarks.The incidence of tattoos in the U.S. and other populations, for example,continues at a significant pace. Because tattoo pigment particles ofabout 1 micron in diameter or less may be cleared from the body viaordinary immune system processes, stable tattoos are likely composed ofpigment particles having diameters on the order of 1-10 microns or more.As the speed of sound in many solid media is approximately 3000meters/second, the acoustic transit time across such particles, andconsequently the laser pulse duration required to achieve theirphotomechanical destruction, is as low as hundreds of picoseconds. Theacoustic transit time of a sound wave in a particle is calculated bydividing the radius of the particle by the speed of sound in theparticle.

In addition to such short pulse durations, high energy laser pulses areneeded for significant disruption of tattoo pigment particles and otherpigmentations. Required fluences of several joules per square centimeterand treatment spot sizes of a few millimeters in diameter translate to adesired laser output with several hundred millijoules (mJ) per pulse ormore. Unfortunately, current systems capable of such short pulseduration and high energy output are too complex and/or expensive forpractical use in the treatment or removal of tattoos and otherpigmentations. These devices generally require two or more lasers andamplifier stages, together with multiple electro-optical and/oracousto-optic devices.

Sierra and Russell (SBIR Proposal to the NIH, submitted December 1993)disclose a device of reduced complexity, which demonstrated 100millijoules of output. The device uses a single laser gain medium thatis common to two resonators. A Pockels cell is used to sequentiallyselect one or the other of the two resonators. Operation requiresapplying a bias voltage to the Pockels cell to establish a modelockedpulse along the first resonator, switching the Pockels cell bias voltageto amplify the pulse along a second, separate resonator, and thenswitching the Pockels cell bias again to extract the amplified pulse.The gain or lasing medium, two polarizers, a Pockels cell, anacousto-optical device, and two mirrors are included along the opticalpathway of the first resonator. The lasing medium, polarizers,electro-optical device, and an additional mirror are included along theoptical pathway of the second resonator.

While this apparatus is less complex than multiple laser systems, itnevertheless requires a large number of optical components (e.g., sevenor more). In addition, the voltages applied and switched at the Pockelscell are equal to the halfwave bias voltage of the Pockels cell,typically in excess of 5,000 volts. These voltages must be switched inless than a few nanoseconds, placing a significant demand on theswitching electronics. Also, because the system utilizes the separateoperation of two resonators, it is possible due to component limitationsfor radiation from one resonator to leak or “spill over” into another. Aconsequence of this is the generation of undesirable or “parasitic”pulses, particularly in the resonator used for amplification, whichsupports a significantly lower threshold for laser oscillation. Finally,the use of an acousto-optic modulator to achieve modelocking may requirethe constant adjustment of resonator length, as such devices operateonly at discrete resonant frequencies.

The simpler alexandrite and other Q-switched lasers currently employedin the treatment of dermatological pigmentations cannot reliably achievetattoo pigment particle clearance in a matter of only a few treatments,despite claims to the contrary. Consequently, there is a need in the artfor laser apparatuses of relatively low complexity, such that they arepractical for tattoo pigment particle removal and the treatment of otherpigmented lesions. Such apparatuses, however, must also be capable ofemitting laser radiation with the short pulse duration required toinvoke photomechanical processes. As discussed above, this requirespulse durations on the order of several hundred picoseconds, or theacoustic transit time across targeted pigment particles. Alsocharacteristic of such a device is the capability of achieving an outputenergy of several hundred millijoules or more.

BRIEF SUMMARY OF THE INVENTION

The present invention is associated with the discovery of methods andapparatuses described herein for delivering pulsed laser energy withpulse characteristics suitable for a number of practical applications.Such pulse characteristics include a sufficiently short duration and/ora sufficiently high energy for the photomechanical treatment of skinpigmentations and pigmented lesions, both naturally-occurring (e.g.,birthmarks), as well as artificial (e.g., tattoos). Advantageously,rather than requiring at least two resonators (or laser cavities),pulsed laser energy having the desired characteristics may be generated,according to a particular embodiment of the present invention, with anapparatus having a single resonator and lasing (or gain) medium,together with an electro-optical device to effect switching between twodifferent operating modes of the single resonator.

In addition to requiring only a single resonator and lasing (or gain)medium, apparatuses may be further simplified in that, in a firstoperating mode, a modelocked pulse is established in the resonator,without the use of an additional modelocking device such as anacousto-optic modulator. Moreover, the need to adjust resonator length,associated with the use of some acousto-optical devices, is eliminated.The overall component and operating requirements of apparatusesaccording to embodiments of the present invention are thereforeconsiderably simplified. For example, in some cases only four opticalcomponents may be required, as is common in many Q-Switched lasersystems.

These and other advantages are associated with the application, to anelectro-optical device (e.g., a Pockels cell) positioned along theoptical axis of the resonator, a time-dependent bias voltage having aperiodic waveform with an amplitude to effect a first operating mode. Inparticular, the periodic waveform has a period substantially equal tothe round trip time of laser energy oscillating in the resonator, whichresults in the generation of a modelocked pulse. Other aspects of thepresent invention include the electronics necessary to generate thetime-dependent bias voltage described above, as well as optionally abaseline operating voltage and voltages for (A) implementing a secondoperating mode of the resonator which amplifies laser energy oscillatingin the resonator and (B) thereafter extracting the amplified laserpulse, having the desired pulse duration and pulse energycharacteristics.

In one embodiment, therefore, the present invention is a method forgenerating pulsed laser energy. The method comprises reflecting laserenergy between two substantially totally reflective mirrors disposed atopposite ends of a resonator and through a polarizer and anelecto-optical device within the resonator and positioned along theoptical path (or longitudinal axis) of the resonator. A lasing (or gain)medium, for example a flashlamp pumped laser rod, is also positionedalong the optical axis. The method further comprises applying to theelectro-optical device a time-dependent bias voltage, V(t), equal to thesum of a baseline voltage, V_(o), and a time-dependent differentialvoltage, δV(t). This time-dependent differential voltage variesperiodically with a period substantially equal to twice the timerequired (i.e., the round trip time) for the laser energy to traversethe length of the resonator, allowing for operation in some caseswithout the need to make adjustments to the resonator length. The methodmay also involve setting or adjusting the amplitude of the timedependent differential voltage and/or pumping the lasing medium (e.g.,using optical pumping means such as a flashlamp) under conditionssufficient to establish a modelocked pulse in the resonator. Thisprovides a first mode of operation in the resonator.

In a subsequent, second mode of operation, the modelocked pulse isamplified. In the case where the electro-optical device is positionedbetween the polarizer and one of the mirrors (arbitrarily denoted the“second” mirror) a first (constant) bias voltage may be applied to theelectro-optical device such that a pulse reflected at this second mirrortraverses the polarizer substantially without loss of intensity oramplitude. To extract the energy from the amplified pulse, a second(constant) bias voltage may thereafter be applied to the electro-opticaldevice such that the polarizer substantially expels a pulse reflected atthe second mirror from the resonator. This releases the pulsed laserenergy having the desired characteristics described herein.

The first bias voltage, for example, may be substantially 0 and thesecond bias voltage may be substantially equal to the quarter wavevoltage of the electro-optical device. The baseline voltage, V_(o), isgenerally from about 30% to about 70%, and often from about 40% to about60%, of the quarter wave voltage of the electro-optical device. Thetime-dependent differential voltage, δV(t), has an amplitude generallyfrom about 5% to about 35%, and often from about 10% to about 30%, ofthe quarter wave voltage of the electro-optical device (e.g., Pockelscell). Advantageously, these voltages are one half or less than thehalfwave voltage (required in known methods) and therefore result in asignificant reduction in the switching electronics requirements.

The pulsed laser energy generated according to methods of the presentinvention may have at least about 100 mj/pulse, and often will have fromabout 200 to about 800 mj/pulse, as required for applications describedherein, such as the removal or dispersion of pigment particles as oftenused to form tattoos. As is also desired in these applications, thepulsed laser energy generally has a pulse duration of at most about 500picoseconds (ps), typically at most about 300 ps, and often at mostabout 150 ps. As stated previously, any of the methods described abovemay be performed without the need to adjust resonator length.

Other embodiments of the invention include laser apparatuses forperforming any of the methods described above, and in particular forgenerating pulsed laser energy with pulses having a duration of at mostabout 500 ps and an energy or intensity of at least about 100 mj. Arepresentative apparatus comprises a resonator having first and secondmirrors, each of which is substantially totally reflective, disposed atopposite ends of the resonator. The apparatus also includes a lasingmaterial (e.g., a solid state lasing medium), an electro-optical device(e.g., a Pockels cell), and a polarizer, all of which are positionedalong the optical axis of the resonator. The electro-optical device ispositioned on this axis between the polarizer and the (arbitrarilydenoted) “second” mirror.

The bias voltage of the electro-optical device may be modified such thattwo operating modes, pulse modelocking and pulse amplification, arecarried out sequentially, as described above, in a single resonator.Therefore, apparatuses according to some embodiments of the invention donot include a modelocking device such as an acousto-optic modulator. Inother embodiments, the apparatuses may include a resonator, and ofteninclude a single resonator only, which is configured to generate laserradiation with the desirable pulse duration and energy characteristicsas discussed herein. The resonator may be included in the apparatus, forexample, in the absence of any other components that would materiallyaffect its basic and novel characteristics.

An additional aspect of the present invention involves the use ofvoltage waveform generating electronics for applying the necessaryvoltage during operation to the electro-optical device, in order toinvoke the separate and normally sequential operating modes of theapparatus, as described above. In particular embodiments, these waveformgenerating electronics apply a time-dependent bias voltage, V(t), equalto the sum of a baseline voltage, V_(o), and a time-dependentdifferential voltage, δV(t). The time-dependent differential voltagevaries periodically with a period substantially equal to the round triptime required for the laser energy in the resonator.

The voltage waveform electronics may be used for initially applying thebaseline voltage, V_(o), to the electro-optical device, prior toapplying the time-dependent differential voltage, δV(t), whichestablishes a first, modelocked operating mode, as discussed above.Subsequently, the voltage waveform electronics can apply a first(constant) bias voltage (e.g., zero voltage or complete discharge of theelectro-optical device), such that a reflected pulse at the secondmirror traverses the polarizer substantially without loss of intensity.Under these conditions the lasing or gain medium amplifies the laserenergy within the resonator, in a second, amplification operating mode,prior to its extraction or release from the apparatus as laser energyhaving the desirable pulse characteristics, including the short pulseduration and high pulse energy discussed above. This generation of suchlaser energy, for applications discussed herein, is ultimately effectedby applying a second bias voltage (e.g., the quarter wave voltage) tothe electro-optical device, such that a pulse reflected at the secondmirror is substantially expelled from the resonator at the polarizer.

In embodiments of the invention, suitable voltage waveform generatingelectronics may include five switches (e.g., MOSFET switchingtransistors S1, S2, S3, S4, and S5, such as those depicted in therepresentative circuit diagram shown in FIG. 4 ) capable of modulatingthe voltage applied to the electro-optical device in a time frame on theorder of 10 nanoseconds. Two high speed diodes and three voltage sourcesmay be used in conjunction with these switches. A first voltage source,for example, may have the capability of applying the baseline voltage,V_(o) (e.g., from about 30% to about 70% of the quarter wave voltage),to the electro-optical device upon closing S1 and S2 and opening S3, S4,and S5. A second voltage source may have the capability of periodicallyapplying the time-dependent differential voltage δV(t) (e.g., having amagnitude from about 5% to about 35% of the quarter wave voltage), suchthat the total bias voltage, V(t), applied to the electro-optical deviceis V_(o)+δV(t). This may be accomplished by closing S1 and opening S4and S5, while periodically opening and closing S2 and S3 with a periodsubstantially equal to the round trip time of the laser energy, in orderto establish the first modelocked pulse operating mode.

Thereafter, the electro-optical device may be discharged by closing S3and S5 and opening S1, S2, changing the value of the effectivereflectivity, R_(eff), of the second mirror to substantially 100%. Thisamplifies the laser energy within the resonator, in a second,amplification operating mode. Extraction or release of the desired laserenergy from the apparatus may be achieved upon closing S1 and S4 whileopening S2, S3, and S5, thereby applying, to the electro-optical device,the voltage differential between two of the three voltage devices, whichis substantially equal to the quarter wave voltage of the device. Thisapplied voltage in turn changes the value of R_(eff) to substantially0%.

In another embodiment, the present invention is a method for treating askin pigmentation, such as a tattoo, a portwine stain, or a birthmark.The method comprises exposing pigmented skin of a patient to pulsedlaser energy with pulses having a duration of at most about 500 ps andan energy of at least about 100 mj. The pulsed laser energy is generatedaccording to any of the methods, or using any of the apparatuses,discussed above.

In another embodiment, the present invention is a method for removing atattoo comprising tattoo pigment particles, which may, for example, havea diameter from about 1 to about 10 microns. The method comprisesexposing the tattoo pigment particles to pulsed laser energy with pulseshaving a duration below about twice the acoustic transit time across thetattoo pigment particles. This pulsed laser energy may have pulses witha duration and energy as described above, and/or may be generatedaccording to any of the methods, or using any of the apparatuses,discussed above.

These and other embodiments are apparent from the following DetailedDescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation showing the relationship betweenthe relative peak pressure within a particle targeted forphotomechanical disruption, as a function of pulse duration, measured asa multiple of the acoustic transit time across the particle.

FIG. 2 depicts a representation of a laser emitting apparatus accordingto the present invention.

FIG. 3A is a graphical representation of voltage applied over time to anelectro-optical device in a laser apparatus, corresponding to a valueV(t)=V_(o)+δV(t) between t₀ and t₁, a value V(t)=0 between t₁ and t₂,and a value V(t) of the quarter wave voltage of the electro-opticaldevice, after t₂.

FIG. 3B is a graphical representation of the effective reflectivity overtime of the combined mirror, electro-optical device, and polarizer inFIG. 2 , with the time-dependent voltage applied to the electro-opticaldevice as shown in FIG. 3A.

FIG. 4 is a schematic of representative waveform generating electronics,capable of delivering the time-dependent voltage to the electro-opticaldevice, as shown in FIG. 3A.

The features of the apparatus referred to in the above FIG. 2 are notnecessarily drawn to scale and should be understood to present anillustration of the invention and/or principles involved. Some featuresdepicted in the figures have been enlarged or distorted relative toothers, in order to facilitate explanation and understanding. The samereference numbers are used in the figures for similar or identicalcomponents or features shown in the various embodiments. Laser devices,as disclosed herein, will have configurations, components, and operatingparameters determined, in part, by the intended application and also theenvironment in which they are used.

DETAILED DESCRIPTION OF THE INVENTION

Aspects of the present invention are associated with the ability oflaser pulses having a duration of several hundred picoseconds to causethe photomechanical disruption, through the use of sound (or pressure)waves, of tattoo pigment particles and other components of pigmentedlesions. Mechanical disruption of the pigment particles facilitatesremoval of the pigment particles by the body's natural removal processessuch as those associated with the immune system. These pulse durationsare of the same order as the acoustic transit time across particleshaving a diameter from about 1 to about 10 microns, which are otherwisesufficiently large to remain stable in skin tissue (e.g., without beingcleared by normal immune system responses).

The significance of short pulse duration in photomechanical processes isillustrated graphically in FIG. 1 , which shows the non-linear responseof peak pressure in a target, as laser pulse duration is reduced. Theunits of pulse duration, along the x-axis, are normalized to a multipleof the acoustic transit time across a targeted particle, such as atattoo pigment particle. The acoustic transit time refers to the timerequired for a sound wave to traverse this target particle. As isapparent from FIG. 1 , the photomechanical stress on the target rapidlyincreases when the irradiating pulse duration decreases to less thanabout two transit times.

The effect becomes dramatically more pronounced below about one transittime. FIG. 1 therefore illustrates the importance of the ability tooperate in the picosecond pulse duration range, in designing aphotomechanical treatment or removal protocol for tattoos and otherpigmented skin lesions. In fact, as is also clear from FIG. 1 , laserpulses having durations of greater than about five times the acoustictransit time induce relatively insignificant peak pressure on the targetparticle and are therefore relatively ineffective in disrupting smallpigmentation particles via the photomechanical mechanism.

Effective apparatuses and methods according to embodiments of thepresent invention are therefore advantageously capable of deliveringlaser energy having a pulse duration generally less than about 1nanosecond, typically less than about 500 picoseconds (ps), and oftenless than about 250 ps. Common pulse duration values according to someembodiments are in the range from about 100 to about 300 ps. The abovevalues generally represent less than several (e.g., from about one toabout three) acoustic transit times for pigmentation particles having adiameter in the range from about 1 to about 10 microns.

Also characteristic of laser energy that is effective for treating orremoving skin pigmentations is a relatively high level of energy output.For example, fluences required to achieve significant disruption ofpigment particles are generally in the range from about 1 to about 10j/cm². For viable treatment methods having a treatment area or spot sizeof a few millimeters in diameter, the required laser output ispreferably at least about 100 mj per pulse, and often in the range fromabout 200 to about 800 mj per pulse.

FIG. 2 depicts a representative embodiment of an apparatus 10 accordingto the present invention, which is capable of achieving the above pulseduration and energy output parameters, suitable for the effectivetreatment of pigmented lesions through photomechanical means.Advantageously, the apparatus includes a resonator (or laser cavity)capable of generating laser energy having the desirable pulse durationand energy per pulse, as described herein. The resonator has acharacteristic longitudinal or optical axis 22 (i.e., the longitudinalflow path for radiation in the resonator), as indicated by the dashedline. Also included in the representative apparatus shown are anelectro-optical device, in this case a Pockels cell 20, and a polarizer18 (e.g., a thin-film polarizer). During operation, the laser pulseoutput will be obtained along output path 23.

At opposite ends of the optical axis 22 of the resonator are a firstmirror 12 and a second mirror 14 having substantially completereflectivity. This term, and equivalent terms such as “substantiallytotally reflective” are used to indicate that the mirrors 12 and 14completely reflect incident laser radiation of the type normally presentduring operation of the resonator, or reflect at least 90%, preferablyat least 95%, and more preferably at least 99% of incident radiation.The mirror reflectivity is to be distinguished from the term “effectivereflectivity,” which is not a property of the mirror itself but insteadrefers to the effective behavior of the combination of second mirror 14,Pockels cell 20, and polarizer 18 that is induced by the particularoperation of the Pockels cell 20, as discussed in detail below.

In particular, a laser pulse traveling from lasing or gain medium 16towards second mirror 14 will first pass through polarizer 18, thenPockels cell 20, reflect at second mirror 14, traverse Pockels cell 20 asecond time, and finally pass through polarizer 18 a second time beforereturning to gain medium 16. Depending upon the bias voltage applied toPockels cell 20, some portion (or rejected fraction) of the energy inthe pulse will be rejected at polarizer 18 and exit the resonator alongoutput path 23. The remaining portion (or non-rejected fraction) of theenergy (from 0% to 100% of the energy in the initial laser pulse) thatreturns to the medium 16 is the “effective reflectivity” of secondmirror 14. As explained above, for any given applied voltage to Pockelscell 20, the effective behavior of the combination of second mirror 14,Pockels cell 20, and polarizer 18 is indistinguishable, in terms oflaser dynamics, from that of a single partially reflective mirror,reflecting the same non-rejected fraction described above. An “effectivereflectivity of substantially 100%” refers to a mirror that acts as asubstantially totally reflective mirror as defined above.

Also positioned along the optical axis 22 of the resonator is a lasingor gain medium 16, which may be pumped by any conventional pumpingdevice (not shown) such as an optical pumping device (e.g., a flashlamp)or possibly an electrical or injection pumping device. A solid statelasing medium and optical pumping device are preferred for use in thepresent invention. Representative solid state lasers operate with analexandrite or a titanium doped sapphire (TIS) crystal. Alternativesolid lasing media include a yttrium-aluminum garnet crystal, doped withneodymium (Nd:YAG laser). Similarly, neodymium may be used as a dopantof pervoskite crystal (Nd:YAP or Nd:YAlO₃ laser) or ayttrium-lithium-fluoride crystal (Nd:YAF laser). Other rare earth andtransition metal ion dopants (e.g., erbium, chromium, and titanium) andother crystal and glass media hosts (e.g., vanadite crystals such asYVO₄, fluoride glasses such as ZBLN, silicaglasses, and other mineralssuch as ruby) of these dopants may be used as lasing media.

The above mentioned types of lasers generally emit radiation, inpredominant operating modes, having wavelengths in the visible toinfrared region of the electromagnetic spectrum. In an Nd:YAG laser, forexample, population inversion of Nd⁺³ ions in the YAG crystal causes theemission of a radiation beam at 1064 nm as well as a number of othernear infrared wavelengths. It is also possible to use, in addition tothe treating radiation, a low power beam of visible laser light as aguide or alignment tool. Alternative types of lasers include thosecontaining gas, dye, or other lasing media. Semiconductor or diodelasers also represent possible sources of laser energy, available invarying wavelengths. In cases where a particular type of laser emitsradiation at both desired and undesired wavelengths, the use of filters,reflectors, and/or other optical components can aid in targeting apigmented lesion component with only the desired type of radiation.

Aspects of the invention also relate to the manner in which therelatively simple apparatus 10, depicted in FIG. 2 , is operated togenerate laser energy with the desirable pulse duration and energyoutput requirements discussed above. For example, laser energy from thelasing medium 16 is reflected between the first mirror 12 and secondmirror 14 at opposite ends of the optical axis 22 of the resonator.Laser energy emanating from the lasing medium 16 therefore traverses thethin film polarizer 18 and Pockels cell 20 before being reflected by thesubstantially totally reflective second mirror 14, back through thePockels cell 20 and polarizer 18.

TIS materials, alexandrite, and other crystals such as Nd:YVO₄ exhibit alarge stimulated emission cross-section selectively for radiation havingan electric field vector that is aligned with a crystal axis. Radiationemitted from such lasing materials is therefore initially linearlypolarized, requiring that the polarizer 18 be configured fortransmission of essentially all incident radiation by proper alignmentwith respect to the electric field vector. However, the application of abias voltage to the Pockels cell 20 can cause elliptical polarization ofthe exiting radiation, such that the radiation field of the pulsereflected in the second mirror 14 and arriving again at the polarizer 18will in this case consist of two components with orthogonal electricfield vectors being out of phase by some angle.

If the polarizer 18 rejects radiation having an electric field vectorthat is orthogonal (or perpendicular) to the orientation of the initialelectric field vector of radiation from the lasing material 16, the neteffect of the combined components (second mirror 14, Pockels cell 20,and polarizer 18) is that of a variable reflectivity mirror. Theeffective reflectivity, R_(eff), of the second mirror 14 (i.e., thePockels cell 20 being positioned between that mirror 14 and thepolarizer 18), is given by equation (1):

$\begin{matrix}{{R_{eff} = {\cos^{2}\left( {\frac{\Pi}{2}V/V_{\lambda/4}} \right)}},} & (1)\end{matrix}$

where the quantity V_(λ/4) is the quarter wave voltage of the Pockelscell 20. The quarter wave voltage refers to the voltage required acrossthe Pockels cell to split the incident radiation into two componentshaving equal intensities and retard the polarization electrical fieldvector of one component by one-quarter of a wavelength relative to theother component.

Thus radiation, having been reflected at the second mirror 14 andtherefore passing twice through the Pockels cell 20 with an appliedvoltage of V_(λ/4), will have its polarization axis rotated 90° and willbe completely rejected by polarizer 18. An applied voltage V=V_(λ/4)therefore provides an effective reflectivity, R_(eff), of “substantially0%,” meaning that the radiation is either completely rejected by thepolarizer 18, or possibly all but a small amount of radiation isrejected (e.g., an amount having an intensity or amplitude generally ofless than about 10%, typically of less than about 5%, and often lessthan about 1%, of its initial intensity or amplitude, I_(o), prior tothe first pass of the radiation through the polarizer 18 and Pockelscell 20). Overall, radiation arriving at the lasing medium 16 after twopasses through Pockels cell 20 (and after having been reflected in thesecond mirror 14) will have an intensity or amplitude, I, given byI=I _(o) ·R _(eff)

It is recognized that, in various embodiments of the invention, thequarter wave voltage can actually induce a number of possible changes inincident radiation polarization, depending on the particular opticalconfiguration of the apparatus. For example, the use of quarter waveretardation plate positioned between Pockels cell 20 and the secondmirror 14 would introduce a double pass polarization axis rotation of90°, without any applied voltage to the Pockels cell. The effectivereflectivity, R_(eff), of the second mirror 14 in this case would begoverned by the expression

${R_{eff} = {\cos^{2}\left\lbrack {\frac{\Pi}{2}\left( {V + V_{\lambda/4}} \right)/V_{\lambda/4}} \right\rbrack}},$where a Pockels cell voltage of 0 would achieve an effectivereflectivity of 0%. Application of the quarter wave voltage to thePockels cell would then introduce an additional 90° of rotation, suchthat the overall effect would be that of no change in polarization. Theeffective reflectivity, R_(eff), in this case would be substantially100%, meaning that the second mirror 14 would act as a substantiallytotally reflective mirror. It is also recognized that not all lasingmedia emit linearly polarized radiation or radiation having an electricfield vector that is aligned with a crystal axis. For example, Nd:YAGmedia are non-polarizing. In the case where non-polarizing media areemployed, polarizer 18 may establish a given polarization of radiationincident to Pockels cell 20.

Various aspects of the present invention are associated with theadvantages obtained when a time-dependent bias voltage, V(t), is appliedto an electro-optical device such as the Pockels cell 20. In preferredembodiments of the invention, the time-dependent voltage is equal to thesum of a baseline voltage, V_(o), and a time-dependent differential oroffsetting voltage, δV(t), that varies periodically with a periodsubstantially equal to the round trip time, or twice the time requiredfor the oscillating laser energy to traverse the length of theresonator. The term “substantially equal” in this case refers todeviations between the period of the applied voltage waveform and theround trip time of generally less than about 100 parts per million(ppm), often less than 10 ppm, and preferably less than about 1 ppm.

The application of a time-dependent voltage waveform described above andcharacterized by equation (2)V(t)=V _(o) +δV(t),  (2)where the time-dependent component δV(t) has a period substantiallyequal to the round trip time of the resonator, allows the resonator tofunction in a first operating mode, where a modelocked pulse isestablished in the resonator. Importantly, modelocked oscillation may beobtained without the requirement for an additional modelocking device(or modelocker), such as an acousto-optic modulator, and consequentlywithout the need to adjust resonator length to match a particularresonance frequency.

Thus, the combination of components, together with the applied voltagewaveform discussed above, can function essentially identically to amodelocker. In the first modelocked pulse operating mode, the effectivereflectivity, R_(eff), of the second mirror 14, is modulated, bymodulating the voltage applied to the Pockels cell 20, with a desiredfrequency (corresponding to a period substantially equal to the roundtrip time of the oscillating laser energy). The modulated reflectivityover time R(t) is obtained by substituting V_(o)+δV(t) from equation (2)into the expression for R_(eff) in equation (1) and expanding to obtain

${{R(t)} = {R_{o} - {2{\cos\left( {\frac{\Pi}{2}V_{o}/V_{\lambda/4}} \right)}{\sin\left( {\frac{\Pi}{2}V_{o}/V_{\lambda/4}} \right)}\left( {\frac{\Pi}{2}\delta{V(t)}/V_{\lambda/4}} \right)} + {\left\lbrack {{\sin^{2}\left( {\frac{\Pi}{2}V_{o}/V_{\lambda/4}} \right)} - {\cos^{2}\left( {\frac{\Pi}{2}V_{o}/V_{\lambda/4}} \right)}} \right\rbrack\left( {\frac{\Pi}{2}\delta{V(t)}/V_{\lambda/4}} \right)^{2}}}},$

where R_(o) is the initial effective reflectivity of the second mirror14. From the above expression, it is evident that when operating atV_(o)=V_(λ/4) or V_(o)=0, the linear term vanishes and modulation of thereflectivity is consequently very small. In contrast, the maximum extentor degree of modulation occurs when the baseline voltage V_(o) is 50% ofthe quarter wave voltage (V_(o)=0.5V_(λ/4)). In preferred embodiments,the baseline voltage V_(o) is from about 30% to about 70%, and typicallyfrom about 40% to about 60%, of the quarter wave voltage of the Pockelscell.

Also, from the above equation for R(t), approximately 30% modulation ofthe reflectivity can be achieved when the magnitude of δV(t),representing either a positive or a negative deviation from V_(o), is20% of the quarter wave voltage. In other embodiments, thetime-dependent differential voltage, δV(t), has an amplitude generallyfrom about 5% to about 35%, and typically from about 10% to about 30%,of the quarter wave voltage of the electro-optical device (e.g., thePockels cell 20). Operation under these parameters, in a firstmodelocked pulse mode of operation, can therefore mimic the operation ofa resonator having an 80% reflecting mirror at one end and alsocontaining a modelocking device such as an acousto-optic device.Modelocking in either case requires a pumping system or device such as aflashlamp (not shown) operating with a sufficient pump rate to thelasing medium 16 to establish the modelocked pulse in the resonator.

In a second (amplification) mode of operation, subsequent tomodelocking, the modelocked pulse generated as described above isamplified. Amplification is achieved by applying a constant (first) biasvoltage to the Pockels cell 20 such that the second mirror 14 has aneffective reflectivity of substantially 100%. In this condition, themodelocked pulse oscillates between two substantially totally reflectivemirrors 12 and 14. In embodiments where the effective reflectivityR_(eff) of the second mirror 14 is governed by equation (1) above, afirst bias voltage of substantially 0 volts (or substantially completedischarge of the Pockels cell), will provide the desired reflectivity ofsubstantially 100%. In this amplification mode, the laser energy canrapidly increase in amplitude by extracting energy that was previouslypumped and stored in the lasing medium 16 during modelocking.

Once the laser energy, oscillating in the resonator under amplificationconditions, has reached a desired or maximum amplitude, it canthereafter be extracted. This is achieved by applying a second biasvoltage to the Pockels cell 20 such that the second mirror has aneffective reflectivity R_(eff) of substantially 0%, to generate pulsedlaser energy. In embodiments where the effective reflectivity, R_(eff),of the second mirror 14 is governed by equation (1) above, a second biasvoltage equal to the quarter wave voltage of the Pockels cell willachieve the desired reflectivity of substantially 100%. At this point,laser radiation having the desirable pulse duration and energy outputdescribed herein, is generated from the apparatus 10 and exits theresonator along output path 23.

FIG. 3A provides a representation of voltage applied, as a function oftime, to an electro-optical device such as a Pockels cell in a laserapparatus, to achieve the operating modes described above. In the timeperiod between to and t₁, the voltage applied is according to theequation V(t)=V_(o)+δV(t), with the time-dependent differential voltage,δV(t), periodically offsetting an applied baseline voltage, V_(o). Inthe particular embodiment of the invention using the voltage waveformshown in FIG. 3A, the baseline voltage is 50% of the Pockels cellquarter wave voltage (V_(o)=0.5V_(λ/4)) and the magnitude of the offsetis 20% of the Pockels cell quarter wave voltage. This offset occursperiodically with a period equal to the round trip time of laser energyin the resonator.

During operation from time t₀ to t₁, the pump rate to the gain or lasingmedium may be set or adjusted to exceed the threshold for laseroscillation, when R_(eff) (the effective reflectivity of the secondmirror) is at or near its highest value. Under these operatingconditions, together with the condition that the period of the appliedvoltage waveform is substantially the round trip time for energy totraverse the resonator as described above, a modelocked pulse can beestablished within the resonator. The time period between t₀ and t₁,where a periodic voltage is applied to the electro-optical device,therefore represents the time that the resonator is operating in afirst, modelocked pulse mode of operation.

At a time t₁, after a steady state modelocked pulse has developed in theresonator, periodic modulation of the applied bias voltage isdiscontinued and a constant (first) bias voltage is then applied to theelectro-optical device, such that R_(eff) is substantially 100%. In theembodiment shown in FIG. 3A, the first voltage, applied at time t₁, is 0volts, meaning that the Pockels cell or other electro-optical device iscompletely discharged. Under this second, amplification mode ofoperation, the amplitude of the laser energy within the resonator isallowed to grow rapidly, drawing upon energy previously input into thelasing medium during pumping in the modelocked pulse operating mode, asdescribed above. When the laser energy has reached a desired amplitude,it may then be released as pulsed energy having the pulse duration andenergy output as described herein. This release is effected by applyinga bias voltage at a later time t₂ such that R_(eff) is reduced tosubstantially 0%. According to the embodiment of FIG. 3A, the appliedbias voltage at this time is substantially equal to the quarter wavevoltage of the electro-optical device.

Amplification and release (or extraction) of laser energy through theapplication of first and second (constant) bias voltages, as describedabove, may also be carried out by applying bias voltages such thatR_(eff) beginning at t₁ is less than 100%. In the amplification mode ofoperation, however, R_(eff) is generally greater than 80%, typicallygreater than about 90%, and often greater than about 95%. Likewise,laser energy may also be released at t₂ using an R_(eff) of greater than0%. For example, a second bias voltage may be applied at t₂ such thatR_(eff) is generally less than 20%, typically less than 10%, and oftenless than 5%. In any event, the important consideration is that thedevice is operated such that R_(eff) is at a relatively high value at t₁and then decreased to a relatively low value at t₂, thereby allowing thedevice to amplify an oscillating laser pulse and thereafter release theamplified laser energy.

In the particular embodiment of the invention characterized by theapplied bias voltage waveform shown in FIG. 3A, the voltage required toobtain an R_(eff) value of substantially 100% at t₁ is substantially 0volts. The term “substantially 0 volts” indicates that theelectro-optical device may be completely discharged to 0 volts or thatthe applied voltage will generally be less than 10%, typically less than5%, and often less than 1%, of the quarter wave voltage of the device.Likewise, in this embodiment of the invention, the voltage required toobtain an R_(eff) value of substantially 0% is substantially equal tothe quarter wave voltage. The term “substantially equal to the quarterwave voltage” indicates an applied bias voltage to the electro-opticaldevice of its quarter wave voltage or preferably at least 80%, typicallyat least 90%, and often at least 95% of its quarter wave voltage.

Also, as explained previously, the Pockels cell or electro-opticaldevice, depending on other components (e.g., a retardation plate) in theapparatus, may require voltages other than 0 and the quarter wavevoltage to achieve R_(eff) values of 100% and 0%, respectively. It isalso apparent from the cyclical nature of the dependency of R_(eff) onthe applied bias voltage, as given by equation (1) above, that highervoltages may be applied to achieve a given effective reflectivity. Forexample, either 0 volts or the half wave voltage may be applied toobtain R_(eff)=100% in equation (1). In general, however, it ispreferred that the smallest bias voltage be applied to achieve a givenR_(eff). Advantageously, the full range of effective reflectivityvalues, from 0% to 100%, may be obtained with the application ofrelatively modest bias voltages in the range from 0 volts to the quarterwave voltage, according to the methods described herein.

FIG. 3B shows, according to one embodiment of the invention, theeffective reflectivity over time corresponding to the time-dependentbias voltage waveform applied to the electro-optical device, as shown inFIG. 3A. During the modelocked operating mode from t₀ to t₁, theeffective reflectivity is periodically and positively offset, from a 50%operating value, to a peak value of 80%. The period of the appliedvoltage waveform matches that of the effective reflectivity waveform,which is the round trip time, or twice the time required for the laserenergy to traverse the length of the resonator. At time t₁ (at thebeginning of the amplification operating mode), when the electro-opticaldevice is discharged, the corresponding value of R_(eff) is 100%. Attime t₂, when the applied bias voltage is V_(λ/4), R_(eff) changes to 0%to release the amplified energy.

The system for generating these waveforms represents another aspect ofthe present invention, as the electronics require not only a peakvoltage of V_(λ/4), but also must be capable of a modulation frequencyof generally at least about 50 MHz, typically at least about 100 MHz(based on a pulse oscillation time on the order of about 10nanoseconds), and often at least about 200 MHz. Values of the modulationfrequency may therefore be within the representative ranges of fromabout 50 to about 200 MHz or from about 75 to about 150 MHz. Inaddition, the switching rise time of the modulation may be approximately1 nanosecond. FIG. 4 depicts one possible type of waveform generatingelectronics for producing the bias voltage and R_(eff) waveforms shownin FIG. 3A and FIG. 3B, respectively. The configuration comprises fiveswitches S1, S2, S3, S4, and S5, meeting the requirements set forthabove. Preferably, insulated-gate, field-effect transistor (IGFET)switches are employed, such as metal oxide semiconductor field-effecttransistor (MOSFET) switches. Two high speed diodes, D1 and D2, andthree voltage sources V1, V2, and V3, are also included, as shown inFIG. 4 .

Also included in the embodiment of FIG. 4 is a Pockels cell 20, to whichthe electronic components apply a time-dependent voltage waveform, suchas that depicted in FIG. 3A. Electrically, the Pockels cell 20 acts as acapacitor, with a typical capacitance of about 10 picofarads (pF). Asdescribed above with respect to FIG. 3A, the waveform generatingelectronics in the embodiment of FIG. 4 are used for a first mode ofoperation at a baseline voltage V_(o) of 0.5V_(λ/4) (or the“eighth-wave” voltage, V_(λ/8)). The baseline voltage is modulated oroffset periodically by the time-dependent differential voltage δV(t)discussed above and having a magnitude of 0.2V_(λ/4) in the particularwaveform shown in FIG. 3A. In a subsequent second mode of operation, thewaveform generating electronics can be used to discharge the Pockelscell (i.e., apply a constant voltage of 0 volts). Thereafter, a voltageequal to the quarter wave voltage, V_(λ/4), of the Pockels cell 20 canbe applied.

In view of FIG. 4 , at time to, the initial bias voltage V_(o) may beapplied from voltage source V1 to the Pockels cell 20 by closing S1 andS2 and opening S3, S4, and S5. Under this condition, the electronicconfiguration shown in FIG. 4 will charge the Pockels cell to theinitial bias voltage V_(o)=0.5V_(λ/4). In a first, modelocked pulse modeof operation between times t₀ and t₁, S1 is maintained closed with S4and S5 open. S2 and S3 are periodically opened and closed with theneeded frequency to modulate the bias voltage (e.g., with a periodsubstantially equal to the round trip time of laser energy in theresonator). In particular, closing S3 while opening S2 modulates thebaseline voltage with the time-dependent differential voltage, δV(t),having a magnitude of offset determined by the voltage from source V2,as shown in FIG. 4 . Opening S3 while closing S2 restores the baselinevoltage (V_(o)=0.5V_(λ/4)) from voltage source V1, through the highspeed diode D2. The total bias voltage, V(t), applied to the Pockelscell 20 is therefore V_(o)+δV(t) during the first mode of operation.

At time t₁, a second, amplification mode of operation is establishedupon closing S3 and S5 and opening S1 and S2. This arrangementdischarges the Pockels cell 20 through S3, S5, and the high speed diodeD1. Finally, at time t₂, closing S1 and S4 while opening S2, S3, and S5applies the quarter wave voltage, V_(λ/4), which is the differentialbetween voltage sources V1 and V3, to the Pockels cell 20, as needed toextract the amplified pulse. Although the Pockels cell capacitance issmall, the switching currents reach several amperes as a result of thevery fast switching times required. Stray capacitance and/or inductancemay impact circuit performance, such that small, tight packaging isdesirable.

Apparatuses and methods disclosed herein can therefore achieve a desiredquality of pulsed laser energy by alternating between two modes ofoperation in a single resonator, rather than through the use of twoseparate resonators. Also, a single Pockels cell, operating in the modesdiscussed above, can eliminate the need for an additional modelockingdevice to establish a modelocked pulse within the resonator. Because thePockels cell does not require operation at a resonant frequency,synchronization with the pulse round trip time is carried out throughsetting the period of the bias voltage modulation, thereby eliminatingthe need to adjust resonator length.

The apparatuses and methods disclosed herein are in many casessignificantly simplified due to the reduced number of components and/orreduced demands in terms of bias voltage and other operating parameters.Devices may be operated using a modulated waveform according to therequirements and parameters set forth herein, and using the electronicconfiguration discussed above or various equivalent configurations aswould be apparent to one of ordinary skill, having the benefit of thepresent disclosure. Other embodiments of the invention may involve theintroduction of conventional optical components for use in conjunctionwith the apparatuses disclosed herein, such as shutters or beamattenuators, reflecting prisms or other reflecting components, filters,light focusing components such as concentrators or condensers,collimating lenses, additional polarizers, electro-optical devices,and/or mirrors, etc. These variations are readily contemplated, and theabove modifications are therefore well within the purview of one orordinary skill, having regard for the present disclosure.

In view of the above, it will be seen that several advantages may beachieved and other advantageous results may be obtained. Various changescould be made in the above apparatuses and methods without departingfrom the scope of the present disclosure. It is intended that all mattercontained in this application, including all theoretical mechanismsand/or modes of interaction described above, shall be interpreted asillustrative only and not limiting in any way the scope of the appendedclaims.

Throughout this disclosure, various aspects are presented in a rangeformat. The description of a range should be considered to havespecifically disclosed all the possible subranges as well as individualnumerical values within that range. For example, description of a rangesuch as from 1 to 6 should be considered to have specifically disclosedsubranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4,from 2 to 5, from 3 to 6 etc., as well as individual whole andfractional numbers within that range, for example, 1, 2, 2.6, 3, 4, 5,and 6. This applies regardless of the breadth of the range.

The following example is set forth as representative of the presentinvention. This example is not to be construed as limiting the scope ofthe invention as other embodiments and aspects of the invention areapparent in view of the present disclosure.

EXAMPLE 1

A laser apparatus as described herein is used to generate pulsed laserenergy having a pulse duration of about 100-200 ps with about 500-750mj/pulse. The laser apparatus includes a resonator with twosubstantially totally reflective mirrors at opposite ends of its opticalaxis. An alexandrite crystal lasing medium, a polarizer, and a Pockelscell are positioned along this optical axis. An optical flashlamp isalso included for pumping the alexandrite lasing medium, which generateslaser energy having a wavelength in the range of 700-950 nm.

The pulsed laser energy described above is generated by pumping thelasing medium and first establishing a modelocked pulse oscillating inthe resonator. In the modelocked pulse operating mode, a time-dependentvoltage waveform, as described herein, is applied to the Pockels cell.This waveform results from the sum of a constant baseline voltage and atime-dependent differential voltage. The baseline voltage is in therange of 1000-1500 volts (representing 40%-60% of the Pockels cellquarter wave voltage, or 2500 volts) and is negatively offset ormodulated by the time-dependent differential voltage, having anamplitude in the range of 250-750 volts (representing 10%-30% of thePockels cell quarter wave voltage). The period of the resulting voltagewaveform is in the range from 5-10 ns and is equal to the round triptime of the oscillating laser energy in the resonator. The voltageapplied to the Pockels cell is thus modulated at a frequency in therange from 100-200 MHz.

Subsequently, the modelocked pulse established as described above isamplified by discharging the Pockels cell to essentially 0 volts.Oscillating laser energy is reflected between the mirrors at each end ofthe resonator, with essentially no losses. This laser energy thereforerapidly increases in amplitude by extracting energy previously pumpedand stored in the alexandrite crystal during modelocking. When the laserenergy has reached the desired energy level as indicated above, it isextracted from the resonator by applying the quarter wave voltage of2500 volts to the Pockels cell.

The switching electronics used to operate the laser in modelocked pulseand amplification modes, and finally to extract the amplified pulse asdiscussed above, comprise five MOFSET switches, two high speed diodes,and three voltage sources having voltages V1 in the range of +1000 to+1500 volts, V2 in the range of +250 to +750 volts, and V3 in the rangeof −1000 to −1500 volts. The switches, diodes, and voltage sources areconfigured as shown in FIG. 4 .

Laser energy having the pulse duration and energy as described above isapplied to a patient undergoing treatment for the removal of a tattoo.This laser energy is applied over the course of a 30-minute treatmentsession to all areas of the skin having undesired tattoo pigmentparticles. Photomechanical disruption of these particles is effectedusing the short pulse duration (below the transit time of a sound wavethrough the targeted tattoo pigment particles), together with a fluencein the range of 2-4 j/cm². This fluence is achieved with a laser energyspot diameter of about 5 mm.

Most if not all of the undesired tattoo pigment particles areeffectively photomechanically disrupted, destabilized, and/or brokenapart using one or two treatments. As a result, the disrupted particlesare cleared from the body via normal physiological processes, such asthe immune response. The tattoo is thus eventually cleared from the skinwith no remaining visible signs. In this manner, various methodsdescribed herein are considered methods for treating or removingpigmented particles such as tattoo particles.

What is claimed is:
 1. A method for treating a skin pigmentationcomprising pigment particles using a photomechanical process, the methodcomprising exposing at least one pigment particle to pulsed laser energygenerated using Nd:YAG, titanium doped sapphire (TIS) crystal, oralexandrite as a lasing medium and with pulses having a duration that isa multiple of the acoustic transit time across said pigment particle tophotomechanically disrupt the pigment particle.
 2. The method of claim 1wherein said pulses have an energy of at least 100 mj.
 3. The method ofclaim 1 wherein said duration is less than 1 nanosecond.
 4. The methodof claim 1 wherein said duration is at most 500 ps.
 5. The method ofclaim 1 wherein said pulses have a duration of between 100 ps and 300ps.
 6. The method of claim 1 wherein said pulses have a duration ofbetween 100 ps and 500 ps.
 7. The method of claim 1 wherein the durationis below twice the acoustic transit time across said pigment particle.8. The method of claim 1 wherein the duration is below five times theacoustic transit time across said pigment particle.
 9. The method ofclaim 1 wherein the duration is from two times to five times theacoustic transit time across the pigment particle.
 10. The method ofclaim 1 wherein the pigment particle is in one of a tattoo, a birthmark,or a pigment particle in a pigmented lesion.
 11. The method of claim 1wherein the at least one pigment particle has a diameter of from 1micron to 10 microns.
 12. The method of claim 1 wherein pulsed laserenergy is generated using Nd:YAG as a lasing medium.
 13. The method ofclaim 1 wherein pulsed laser energy is generated using titanium dopedsapphire (TIS) crystal as a lasing medium.
 14. The method of claim 1wherein pulsed laser energy is generated using alexandrite as a lasingmedium.